1893.] THE MICROSCOPE. 7 



again, any one will learn after repeated trials the exposure 

 necessary, being guided by the intensity of light upon the ground 

 glass. This varies with different plates and is another reason 

 why one kind of plate should be adhered to. 



There seems to be a variety of opinion with regard to the use 

 of an ocular. Personally I do not use the ordinary Huyghe- 

 nian ocular in photomicrography except in focusing as hereafter 

 illustrated. Oculars especially corrected for this work may be 

 obtained, but unless one is at hand I have found it best to use only 

 the objective, which, if a good one, will give good results. I am 

 aware, however, that there are those who hold a different opinion 

 upon this point. 



Microscopical enlargements are often, though improperly, ex- 

 pressed in " times" when diameters are intended. It is evident 

 that if an object one-thousandth of an inch in diameter be made 

 to appear one inch in diameter it has been magnified one thousand 

 diameters, while the area has been increased one million times. 



A few words in regard to the object to be photographed may 

 not be amiss here. As we always use light transmitted through 

 the object, it is obvious that objects which produce the greatest 

 contrast in lights and shadows are the best for our purpose. The 

 picture, while possessing little or no perspective, may show a 

 great amount of detail. 



The coloring of the object is another important point to con- 

 sider. Sections stained with brown, black, or Heidenhains' im- 

 pregnation give finer results than those stained with picro-carmine 

 or blue. The part of a section to be photographed should be 

 carefully selected by direct examination. It should be thin 

 enough to show the desired details plainly, and, if possible, the 

 coloring should give a good contrast of lights and shadows. 



For all objectives of less power than a one-fourth or one-fifth, 

 if the object be illuminated by sunlight, I would not use a sub- 

 stage condenser, but for a one-fifth or higher power I would use it. 



As has been said, sunlight is the best source of light ; electricity 

 and the magnesium light have been employed, but for the majority 

 of workers they are not practicable. In using sunlight a heliostat 

 is a luxury which most cannot afford, but it is a convenience 

 which all who attempt photomicrography, using solar light, would 

 appreciate. Evidently the light must be reflected, and a mirror 

 mounted upon a support so that it can be readily turned in any 

 direction is a very nice adjunct to the opposite end of our camera 

 board. 



This can be easilv arranged as follows : Remove the mirror 

 and support from the mirror-bar and make a clasp of a piece of 

 brass three inches long, three-eighths of an inch wide, and 

 one-fourth of an inch thick by turning up one-half inch on 

 either end. In one end fit a small screw that will bear 



